Heat & Altitude Performance Calculator
Adjust race pace targets for temperature, humidity and elevation
Heat & altitude
Race pacing targets adjusted for heat, humidity and altitude.
Based on Ely et al. (2007) — ~0.5% slowdown per °C above 15°C
How the heat and altitude calculator works
Heat and altitude both slow you down, and both by amounts you can plan for. In the heat, blood is diverted to the skin to shed the load, less reaches the muscles, and sustainable pace falls — gently at first, then steeply as the wet-bulb temperature climbs, because humidity stops sweat from cooling you. At altitude the air carries less oxygen, and every aerobic effort above the lactate threshold is slower for it.
Enter the temperature, humidity and elevation of the race and the calculator adjusts a target pace or power to the conditions. The loss is a percentage that grows with the effort duration — a marathon suffers more from 28 °C than a 5 km does — and, for altitude, with the height and how little time you have had to acclimatise.
The correction says what the day is worth, so you set out at a pace the conditions allow rather than the one your fitness would allow at 15 °C and sea level. Going out at the flat-and-cool target in the heat is the most common way a well-prepared race is lost.
Questions
- How much slower is a marathon in the heat?
- From about 15 °C upward the loss grows roughly with every degree, and faster once humidity is high: 25 °C and humid typically costs a marathon runner several percent, and 30 °C with high humidity well over that. Shorter races lose less because the body has less time to heat up.
- Does altitude affect short races too?
- Less. Sprints and efforts under a minute barely notice it; anything aerobic does, and the longer the race the more. Above about 1500 m the loss becomes noticeable, and the first days are worst — acclimatising for two to three weeks recovers part of it.
- Can I train in the heat to blunt the effect?
- Yes. Ten to fourteen days of training in the heat raises plasma volume and sweat rate and lowers the cost of a given pace in hot conditions, with the effect lasting a couple of weeks after. It does not remove the loss, so still adjust the target.
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